IQ modulation is a very general form of modulation that allows independent influence of both the amplitude and phase of the carrier signal being modulated. Although IQ modulators can be used in a truly continuous analog manner, they are widely used in modulating carrier signals with program material that is either in a digital format to begin with, or that has been digitized prior to modulation. In digital modulation various combinations of carrier amplitude and phase are assigned respective digital values. For example, if eight amplitudes were allowed, and eight phases, the transmitted carrier would have a repertoire of sixty-four ‘modulation symbols’ that would be placed into correspondence with, say, sixty-four different six-bit integers. This is a process of encoding, particularly if the mapping is not static and not altogether obvious (of which complexity there are well known standard techniques). Program material to be modulated in this manner is rendered into a stream of bits, grouped into six-bit bytes, and those bytes are applied to the encoder/modulator at some convenient rate. The actual I and Q input signals to the modulator are frequently filtered to limit their slewing rate as they go from one value to the next, the better to keep spurious sidebands from being objectionable. At the receiving end a demodulator/decoder recognizes (according to the mapping) the different combinations of amplitude and phase as the same modulation symbols, and re-produces a sequence of the corresponding (original) bytes, which are then reassembled back into the original bit stream of digital program material. In many applications, the recovered bits are then appropriately framed into words and applied to an ADC (Analog to Digital Converter) to reproduce analog program material for human consumption (e.g., a voice channel for a cellular telephone).
Fidelity in this process depends upon, among other things, that the IQ modulator accurately produces the various modulation symbols (combinations of carrier amplitude and phase). In a truly digital setting or application there is little or no restriction on what modulation symbol can follow another (just as there is no restriction on what bit can follow another in an arbitrary bit stream), so there is little or nothing that the demodulator per se can do to detect errors. True, there are systems that use training sequences to characterize a channel, and such things as error correcting codes for expressing the program material. These, however, are levels of abstraction and control that are wrapped around, or built on top of, the IQ modulation scheme. The I/Q modulator/demodulator pair must deal with those ‘special’ bits, just as it would with any ‘ordinary’ bits. The first line of defense for system integrity is the accurate production of the correct IQ modulation symbols, as close to their intended parametric values as possible (i.e., on frequency, with the correct amplitude and the correct phase). It is in this way that the demodulator/decoder has the best chance of sending/reproducing the original program material.
Each IQ modulator has a collection of one or more operational control parameters that can be adjusted over associated ranges, and whose combination of values (if there should be more than one control parameter) produces modulator operation that approaches the ideal in greater or lesser degrees.
It turns out that, especially when used at VHF (Very High Frequencies) and above, say above 100 MHz, that two IQ modulators that are ostensibly identical assemblies will produce slightly different parametric values for their modulation symbols. Exact measurement of this is a difficult task for general purpose test equipment, such as counters and spectrum analyzers, and the better instances of such general purpose test equipment that have the required speed and resolution are fairly expensive. They can sometimes be fussy to set up and make measurements with confidence, particularly for operators who are less familiar with their operation. Special purpose modulation analyzers are available whose measurement capabilities are deliberately tailored for, and thus more suited to, IQ modulation of the various popular flavors, but they are not inexpensive, either! Furthermore, while either of those measurement strategies will distinguish between ‘generally good’ and ‘truly bad’ behavior, it is not clear that either can be easily used to assist in finding or recognizing operating points for the applied control parameters that result in ‘improved’ behavior.
To improve the fidelity and reliability of an overall IQ modulation system it would be desirable if there were a cost effective way of characterizing a particular instance of an IQ modulator so as to optimize its actual behavior by having it produce the best possible approximation of ideal behavior. This is particularly so if the modulator is part of an integrated circuit assembly that is otherwise to be tested on automated IC tester that does not normally include the measurement repertoire of a genuine modulation analyzer.